A physical system which can be forced to execute an arbitrary unitary transformation, and its use to perform arbitrary tests

It is shown that a system of N spin‐(1/2) magnets can be constructed so freely manipulable that an experimenter can impose any unitary transformation upon its state space, N any positive integer. This is then used with N ≥ b − 1 + 2 log2 n to construct arbitrary states over an n‐dimensional Hilbert space and to perform arbitrary b‐tests upon these states.